Conventionally, a chromate treatment using hexavalent chromium has been used to improve the corrosion resistance of aluminum, magnesium, zinc, and zinc alloys. However, because of the harmfulness to the human body, the use of hexavalent chromium has been regulated. For this reason, a chemical conversion treatment using trivalent chromium has been used as an alternative to the chromate treatment. Moreover, to improve the corrosion resistance and the scratch resistance, atop coat may be applied on a chemical conversion film or on a finished surface prepared by subjecting a chemical conversion film to a finishing treatment as described in, for example, Japanese Patent Application Publication No. 2005-23372, WO2012/137680, etc. In the top coating treatment, a polyolefin wax-based friction modifier is generally mixed and dispersed in a top coating agent to adjust the friction coefficient. However, an acidic water solvent-based top coating agent, which is generally used for a trivalent chromium chemical conversion film or a chromium-free chemical conversion film, has such a problem that the dispersed polyolefin wax separates, and hence a stable friction coefficient cannot be obtained.